1. Field of the Invention
This invention relates to an imager using Focal Plane Arrays (FPAs), in particular imaging in the Infra Red (IR) wavebands using such devices.
2. Description of the Prior Art
IR imaging systems are becoming more important in many fields now, in particular military, security and search and rescue applications. Early IR imagers employed a small number of detector elements, across which was scanned an IR image of the scene via a system of mirrors and polygons. More recent developments include imagers based on 2 dimensional arrays of detector elements, so called staring arrays, which require no scanning to produce a useful image of the scene. The dwell time available for each detector element in such systems is considerably increased over earlier scanner systems resulting in significantly improved system performance being achievable from comparable detector materials. The IR system designer can choose whether to exploit this increase in performance or use a lower performance detector material to achieve a similar sensitivity as in the earlier scanner systems. High system performance is typified by imagers based on arrays of Cadmium Mercury Telluride cooled to liquid nitrogen temperatures, whilst conventional levels of performance are achieved by imagers based on Schottky barrier arrays and pyroelectric ceramics. These latter systems offer significant advantages in terms of cost and/or logistical support requirements (such as coolant supplies) over the high performance systems.
Unfortunately, several disadvantages of FPA imagers must be overcome to provide performance comparable with conventionally scanned imagers. Current FPAs are only available in limited pixel counts, typically 128.times.128 or 256.times.256 elements, which is insufficient to match the spatial resolution of the best scanned imager systems. Eventually, the development of suitable fabrication technologies will overcome this problem, resulting in large pixel densities.
At the present time however, staring focal plane array imagers require the image to be microscanned, or dithered, on the focal plane for full spatial resolution to be achieved from the system. The image formed must be constructed from interlacing several such microscanned fields to produce a composite frame of data before display. The general concept for a 2.times.2 microscan system is shown in FIG. 1. The image is moved through 4 positions A to D, 2 in each axis, and interrogated by the FPA at each position. The full frame of data E, comprising the suitably interlaced microscan fields, is then displayed to the observer with the full resolution of the system. Although FIG. 1 shows a 2.times.2 microscan approach, other relationships are possible, such as 2.times.1 in a diagonal axis, 2.times.3 and 3.times.3. The optimum number of fields used to produce the composite frame depends on several factors including the detector element size and shape, the elemental pitch, and the relative Modulation Transfer Functions (MTFs) of the other components, such as the optics, in the system.
An imager employing an improved microscanning technique, (which is the subject of a copending application having the same date of filing as the present application, Ser. No. 07/748,812) is disclosed in FIG. 2.
The imager 1, comprises a lens 2, a two dimensional sensor array 3, and chopper 4 driven by a motor 5. The chopper is shown in plan view in FIG. 3 and comprises a plurality of non transmissive regions 5 and transmissive regions 6 comprising refractive material angled so as to refract radiation in the directions indicated by arrows 7.
When the motor 5 of FIG. 2 is energised by battery 8 the chopper 4 rotates, and each line of the sensor array 3 is read out by the electronic circuit 9 as the leading edge 10 of each non transmissive region 5 passes over it.
Incident radiation on the imager is refracted in a different direction by each successive transmission region 6 of the chopper 4 thereby implementing microscan. Successive frames are interleaved digitially in a frame store within the electronic circuit 9, prior to display on the cathode ray tube 11. This results in several disadvantages of the overall system.
1. The system can be quite electronically complex and requires frame storage.
2. The system complexity results in inevitable system expense.
3. By interlacing the data in a frame store before subsequent display in a single frame, the temporal continuity of the imaging process is destroyed, resulting in a loss of the benefits of microscan when the image is moving. This is particularly so when the imager is panned, resulting in multiple images being produced each corresponding to the position of the image o the detector in each of the microscan fields. Since these are essentially uncorrelated due to the imager motion, extensive data manipulation would be required to overcome the problem.
4. In general such systems utilise a cathode ray tube (CRT) for the display of information to the user. Such displays, although commonly used, suffer from considerable disadvantages. In particular, the CRTs are manufactured using vacuum glass technology, and as such are particularly fragile unless steps are taken to ruggedise the tube. Also likely to suffer damage from vibration and shock are the delicate electrodes and phosphor screen coatings. Although ruggedised CRTs are available they are expensive and more bulky than their conventional equivalents. In addition, the power consumption of CRTs is usually in excess of one watt, and can run to several watts for the larger and/or ruggedised tubes, not including the requirements of the drive electronics. A further problem associated with CRT displays is that the electronic drive circuits rarely operate with the same scan sequences as the detector, resulting in the requirement for additional frame storage and data resequencing--if only at different timings-- between the detection and display processes.
An image overcoming the need for a cathode ray tube to display an image (which is the subject of another copending application having the same date of filing as the present application, Ser. No. 07/748,212) is disclosed in FIG. 4.
The imager of FIG. 4 comprises a lens 20 focusing radiation onto a sensor array 21. In operation radiation incedent on the sensor array 21 is interrupted by a chopper 22, which is shown in greater detail in FIG. 5. The chopper is driven by motor 23, energised by battery 24, and is synchronised to an electronic circuit 25, which reads each frame of the sensor array 21 sequentially line by line, as each leading edge 26 of each chopper blade 27, passes over it. The electronic circuit 25 energises a linear array of LEDs 28 such that they are illuminated in dependence upon the radiation received by each corresponding element of the line being read from the sensor array 21. A reflective surface 29 of a polygon 30 driven by the motor 23, scans radiation from the array of LEDs such that, due to natural persistence of vision, an image apparently comprising a two dimensional array of LEDs depicting the image received by the sensor array 21, is seen by an operator.
The present invention arose from the realisation that the principles of the two inventions above described could be employed to provide an improved imager.